Current implementation is support only isolated borders. We should support also non-isolated borders (when we can look outside of src roi upto whole src image).

For isolated mode: roi for src = dst roi. We don't look outside of src roi.
For non-isolated mode: roi for src is a **whole** src image. We look outside of src roi up to {0,0,wholeSrcWidth, wholeSrcHeight} (srcoffset should not be used here).

All current tests use non-isolated borders only (BORDER_ISOLATED bit is off).
In OpenCV all borders are not isolated by default.